The fish family Amblyopsidae are commonly referred to as cavefish, blindfish, or swampfish. They are small freshwater fish found in the dark environments of swamps, caves and deep lakes. Family members exhibit adaptations to these dark environments, including the lack of functional eyes and, in some species, the absence of pigmentation. There are more than 80 known varieties of cavefish.^[1]

Description

Cavefish are probably ancient in origin. Adaptations common to many cavefish include reduced susceptibility to light, pigment loss, or reduction in skin scales,^[1] as well as development of chemoreceptors in the sensory organs of the body surface and the lateral line. Three species exhibit efficient metabolism during swimming, as compared with a group of non-cave fish,^[1] and many cavefish species exhibit slender bodies adapted to swimming in fast-flowing waters.^[1]

Their bodies lack pigment and are translucent. They have a naked, moderately depressed head and an elongated body, covered with small cycloid irregular flakes, with tiny or absent pectoral fins. The anal opening is so far forward that is in the throat region.

The premaxilla, a bone of the upper jaw, is segmented, the vomer has no teeth. They have no ventral fins. The lateral line is incomplete, but well developed in some species. Its spine has between 27-35 vertebrae.

Externally, they resemble killfishes in many respects, although their internal anatomy more closely resembles the trout-perches, with which they are currently classified.^[2]

Cavefish have an unusual body clock that does not respond to light. The period of their circadian rhythm has grown to 47 hours.^[3][4]

Eyes

The fish of this family are called amblyopsis because they are either blind or can only detect the difference between light and darkness. The true cave-dwellers have only rudimentary eyes, like so many other fauna who live in the dark.

Although some species have tiny, vestigial but functional eyes, others, such as the springfish and the ricefish have no eyes at all. Blindfish do, however, have rows of sensory papillae on their skin, which they use to help navigate.

Similar darkness-adaptive traits can be seen in fish families in other types of isolated environments. This is a manifestation of the phenomenon known as convergent evolution, and has been observed in Characidae, Cyprinidae, Gobiidae, Ostariophysi, Poeciliidae, Synbranchidae and to the freshwater and marine fish group Bythitidae.

Distribution

All cavefish are small and typical of the fresh waters of the southern regions of North America. Some live deep in the swamps, and others in the lakes and streams or in caves, significantly the Kentucky cave system called the Mammoth caves.

Cavefishes can only be found in caves that have streams running into them; a cave with no inlets, such as Blanchard Springs Caverns in Arkansas, does not contain cavefishes.

Ecology

Although the cave habitat generally offers a poor food supply, the advantages of the environment include extremely stable conditions, few competitors and few predators.^[1]

Since the cave environment is dark, there is no plant life performing photosynthesis, and food is mainly introduced from the outside world by other organisms. Limited food leads to low population density, which has been estimated for Amblyopsidae to be only about 0.005 to 0.15 animals per square meter.^[1]

Life history/behavior

Cavefish breed only once a year, occupying about ten percent of the population.^[1] The eggs are large and are protected for four to five months over the gills of the female cavity. Cavefish protect their eggs for the longest period of any fish.^[1]

Notable Species

One species, the swampfish Chologaster cornuta, inhabits above-ground swamps, rather than caves.^[2] A rare feature of this family is the forward placement of its cloaca, under the head, anterior to the pelvic fins.^[2] This placement allows the females to place their eggs more precisely. This feature is present also in others species of the Percopsiformes order, like Aphredoderidae. They feed on shrimp, gammarus and arachnids that fall into the water using vibrations and current changes to seek out their prey.

The blindfish species are blind and depigmented. Despite lack of eyes, blindfish do not exhibit nyctophilia. Blindfish are generally small, ranging up to 11 centimetres (4.3 in) in length. Most do not have pelvic fins.

Typhlichthys subterraneus is mostly lentic, but can also be found in pools of streams. This blindfish feeds mostly on aquatic arthropods, such as amphipods and isopods. Their metabolic rates are depressed in order to survive food shortages.^[5] Females produce fewer than 50 eggs per breeding cycle. This restricts its ability to recover from an even minor population decline. They young brood in the female's gill chamber. Sexual maturity requires approximately two years. The life span of blindfish is approximately four years.^[6]

The Forbesichthys agassizii has dark brown dorsals and fades to a creamy brown towards the pelvis. It feeds primarily on amphipods, such as Gammarus troglophilus, but also prey on midge larvae and annelid worms.

The Amblyopsis rosae lives in caves that have a relatively large source of nutrients, such as bat guano or blown leaf litter. Water quality in caves that contain cavefish is usually high. Ozark cavefish tolerate extremely low oxygen content.

Cavefish tend to occur in flowing cave streams as opposed to quiet pools. Ozark cave fish can receive nutrients without photosynthesis or chemosynthesis from tree roots above the cave. The roots are full of nutrients and water.

Amblyopsis spelaea has small pelvic fins with up to six rays. They are predators absent edible vegetation. They inhabit subterranean water or caves which have consolidated mud-rock substrates in shoals and silt-sand substrates in pools. It is more often found in caves with uniform silt-sand substrates. Amblyopsis spelaea have low reproduction rates and broods in its gill cavity for up to four to five months. Its low metabolism allows it to live for two years without food.

Relation to humans

Cave habitats are vulnerable to changes in the environment such as water pollution and exotic species.^[1] The World Conservation Union (IUCN) has placed four cavefish species on the Red List of threatened species.^[1] Alabama cavefish, Speoplatyrhinus poulsoni, which live only in certain caves in Alabama, are listed as Critically Endangered, the highest risk class.

References